Researchers extracted and examined cores of sediment from a Caribbean lagoon in order to look for evidence of past storm surges from intense hurricanes. (Photo courtesy of Jeff Donnelly, Woods Hole Oceanographic Institution)

Reconstruction Reveals that El Ni?o and the West African Monsoon Have Strongly Influenced Intense Hurricane Frequency

The frequency of intense hurricanes in the Atlantic Ocean
appears to be closely connected to long-term trends in the El Niño/Southern
Oscillation (ENSO) and the West African monsoon, according to new research from
the Woods Hole Oceanographic Institution (WHOI). Geologists Jeff Donnelly and
Jonathan Woodruff made that discovery while assembling the longest-ever record
of hurricane strikes in the Atlantic basin.

Donnelly and Woodruff began reconstructing the history of
land-falling hurricanes in the Caribbean in 2003 by gathering sediment-core
samples from Laguna Playa Grande on Vieques (Puerto Rico),
an island extremely vulnerable to hurricane strikes. They examined the cores
for evidence of storm surgesdistinctive layers of coarse-grained sands and
bits of shell interspersed between the organic-rich silt usually found in
lagoon sedimentsand pieced together a 5,000-year chronology of land-falling
hurricanes in the region.

In examining the record, they found large and dramatic
fluctuations in hurricane activity, with long stretches of frequent strikes
punctuated by lulls that lasted many centuries. The team then compared their
new hurricane record with existing paleoclimate data on El Niño, the West
African monsoon, and other global and regional climate influences. They found
the number of intense hurricanes (category 3, 4, and 5 on the Saffir-Simpson
scale) typically increased when El Niño was relatively weak and the West
African monsoon was strong.

“The processes that govern the formation, intensity, and
track of Atlantic hurricanes are still poorly understood,” said Donnelly, an
associate scientist in the WHOI Department of Geology and Geophysics. “Based on
this work, we now think that there may be some sort of basin-wide ‘on-off switch’
for intense hurricanes.”

Donnelly and Woodruff published their latest results in the
May 24 issue of the journal Nature.

Donnelly and his colleagues have pioneered efforts to extend
the chronology of hurricane strikes beyond what can be found in historical
texts and modern meteorological records and previously applied their methods to
the New England and the Mid-Atlantic coasts of the United States.

Their research area, Laguna Playa Grande, is protected and
separated from the ocean during all but the most severe tropical storms.
However, when an intense hurricane strikes the region, storm surges carry sand
from the ocean beach over the dunes and into Laguna Playa Grande. Such
“over-topping” events leave markers in the geological record that can be examined
by researchers in sediment core samples.

The geological record from Vieques showed that there were
periods of more frequent intense hurricanes from 5,000 to 3,600 years ago, from
2,500 to 1,000 years ago, and from 1700 AD to the present. By contrast, the
island was hit less often from 3,600 to 2,500 years ago and from 1,000 to 300
years ago.

To ensure that what they were seeing was not just a change
in the direction of hurricanes away from Viequesthat is, different storm
tracks across the Atlantic and Caribbeanthe scientists compared their new
records with previous studies from New York and the Gulf Coast. They saw that
the Vieques record matched the frequency of land-falling hurricanes in New York and Louisiana,
indicating that some Atlantic-wide changes took place.

Donnelly and Woodruff, a doctoral student in the MIT/WHOI Joint
Graduate Program, then decided to test some other hypotheses about what
controls the strength and frequency of hurricanes. They found that periods of
frequent El Niño in the past corresponded with times of less hurricane
intensity. Other researchers have established that, within individual years, El
Niño can stunt hurricane activity by causing strong winds at high altitudes
that shear the tops off hurricanes or tip them over as they form. When El Niño
was less active in the past, Donnelly and Woodruff found, hurricane cycles
picked up.

The researchers also examined precipitation records from
Lake Ossa, Cameroon, and discovered that when there were increased monsoon
rains, there were more frequent intense hurricanes on the other side of the
Atlantic. Researchers have theorized that frequent and stronger storms over
western Africa lead to easterly atmospheric waves moving into the Atlantic to provide the “seedlings” for hurricane
development.

Much media attention has been focused recently on the
importance of warmer ocean waters as the dominant factor controlling the
frequency and intensity of hurricanes. And indeed, warmer sea surface
temperatures provide more fuel for the formation of tropical cyclones. But the
work by Donnelly and Woodruff suggests that El Niño and the West African
monsoon appear to be critical factors for determining long-term cycles of
hurricane intensity in the Atlantic.

The research by
Donnelly and Woodruff was funded by the National Science Foundation, the Risk
Prediction Initiative, the National Geographic Society, the WHOI Coastal Ocean
Institute, and the Andrew W. Mellon Foundation.

The Woods Hole Oceanographic Institution is a private,
independent organization in Falmouth, Mass., dedicated to marine research,
engineering, and higher education. Established in 1930 on a recommendation from
the National Academy of Sciences, its primary mission is to understand the
oceans and their interaction with the Earth as a whole, and to communicate a
basic understanding of the ocean's role in the changing global environment.

Originally published: May 23, 2007

WHOI is the world's leading non-profit oceanographic research organization. Our mission is to explore and understand the ocean and to educate scientists, students, decision-makers, and the public.